Isildur's Bane

Equip {5}
Equipped creature gains unblockable and gets +2/+2 for each power counter on it.
At the beginning of your upkeep, put a power counter on equipped creature. Then if there are 3 or more power counters on it, destroy that creature and put Isildur's Bane on the top of its owners library.
If this card is equipped to a non-legendary creature, destroy that creature and return Isildur's